This invention relates to controlling the charging of electrical storage batteries, and, more particularly, to attaining optimal charging of sodium-sulfur batteries having multiple storage cells.
Rechargeable storage cells are electrochemical devices for storing and retaining an electrical charge and later delivering that charge as useful power. A number of such storage cells are typically connected together electrically to form a battery having specific voltage or current delivery capability. Familiar examples of the rechargeable storage cell are the lead-acid storage cell used in automobiles and the nickel-cadmium storage cell used in portable electronic devices such as cameras. Another type of storage cell having a greater storage capacity for its weight is the nickel oxide pressurized hydrogen cell, an important type of which is commonly called the nickel-hydrogen cell and is used in spacecraft applications.
Yet another type of storage cell is the sodium-sulfur storage cell, which has been under development for over 20 years for use in a variety of terrestrial applications such as nonpolluting electric vehicles. The sodium-sulfur storage cell has the particular advantage that its storage capacity per unit weight of storage cell is several times the storage capacity of the nickel-hydrogen cell. The sodium-sulfur storage cell therefore is an attractive candidate for use in spacecraft applications as well as automotive applications.
The most common type of construction for a sodium sulfur storage cell includes a cylindrical metal outer housing which serves as a positive terminal and a cylindrical shell of an alumina-based ceramic within the outer housing. Sodium is placed into a first or inner chamber formed within the alumina shell, and sulfur is placed into a second chamber formed between the alumina shell and the outer housing. The storage cell is heated to a temperature of about 350 C., at which temperature both the sodium and the sulfur are molten. The liquid sodium is the anode of the storage cell, the liquid sulfur is the cathode, and the solid ceramic is the electrolyte. Electrical energy is released when sodium ions diffuse through the ceramic into the sulfur, thereby forming sodium polysulfides. Electrical energy can be stored when the process is reversed during charging of the battery, with an applied voltage causing the sodium polysulfides to decompose to yield sodium and sulfur, and the sodium ions diffuse through the ceramic electrolyte back into the first chamber.
It is observed in some tests of batteries made from a number of individual sodium-sulfur storage cells connected in series that the usable electrical storage capacity of the battery may be less than expected, and may deteriorate even further after repeated cycles of charging and discharging. This reduced storage capacity must be considered in designing the battery and assessing the total battery capacity required to provide sufficient storage capacity for a particular mission. Since the sodium-sulfur battery may be required to operate for 10-20 years, even very gradual reductions in storage capacity may have a marked adverse impact on spacecraft design by adding considerable battery weight, because the battery must be designed to meet the performance requirements at the end as well as the beginning of the mission.
There is a need for an improved sodium-sulfur storage battery which has the expected storage capacity and retains that storage capacity over extended periods of time. The present invention fulfills this need, and further provides related advantages.